This invention relates to shape retaining bodies of solder that are useful for joining two solderable surfaces.
Solder preforms have been used for many years to ease the assembly of solderable surfaces. Some initial approaches used an infusible fixture to hold the solder in place while the joint was made. U.S. Pat. No. 2,055,276 teaches the use of an infusible circular metal channel containing solder to join plumbing pipes and fittings. This infusible channel is required to hold the solder in place while it is reflowed and the joint is formed. Unfortunately, this infusible channel has inherent disadvantages. The lack of conformability of this hard, infusible, metal channel makes aligning the solderable surfaces more difficult. In addition, this infusible channel becomes a permanent but non-load-bearing, part of the joint. This infusible preform also complicates the application of flux, which is necessary to make a good joint.
Flat solder preforms are also used in other applications where the preform can be laid horizontally on the surface to be soldered. In these instances, the reflowing solder simply flows down onto the solderable surface and the joint is made. U.S. Pat. No. 4,020,987 teaches the use of a solder preform for hermetic package sealing. Likewise, U.S. Pat. No. 4,709,849 teaches the use of specific-shaped solder preforms to control the amount and location of solder for the soldering of electronic components. In all these cases, gravity requires that the solderable surfaces be horizontal.
U.S. Pat. No. 1,947,581 teaches an alternate approach to controlling the location of the solder during the flow process. This patent teaches creating channels of specific dimensions in the solderable surfaces. These channels are reported to control the location and amount of solder consumed in the soldering process.
For many years it has been known that flux is required to ensure the solder will bond or alloy adequately to the solderable surfaces. Unfortunately, delivering the correct amount of flux to the correct location such that it can prepare and protect the solderable surface for soldering prior to the solder melting is difficult. For pipe applications, the solderable surfaces are typically coated with flux prior to soldering. The two solderable surfaces are then assembled. Typically, solder in the form of wire or strip is melted near the exposed, outer end of the joint such that capillary forces draw the molten solder into the gap between the two solderable surfaces. This two step process is effective but very slow. In addition, because the operator can not see the inner end of the joint being made, excess or insufficient amounts of both flux and solder are frequently used. Typically, excess molten material flows through the solderable gap and out the opposite end of the joint. If insufficient solder and flux are used, the joint is not sound.
To ensure a good joint between the solderable surfaces, flux should be used. To be effective, the flux must be applied to the solderable surfaces prior to the application or installation of a solder preform. Solder performs are shaped solder structures designed to simplify the solder application step of the conventional soldering process. However, the use of solder preforms does not adequately address the application of flux.
The problem of applying flux to solderable surfaces has been the subject of many development efforts and some more recent patents. One common flux-containing solder preform is a material often called rosin-core solder wire. This preform is in the shape of a hollow wire with the flux contained therein. Unfortunately because this preform is not in the shape of the solderable surfaces to be joined, it provides no means of insuring that the correct amount of solder and flux are delivered to the joint. In plumbing applications, this flux-containing solder wire is typically melted and then pushed into the joint. Because the operator has no way of knowing how much solder to push into the joint, the quality of the resulting joint is often inconsistent. And in many instances, excess solder accumulates inside the pipe in contact with the fluid which passes through.
Several inventions have attempted to incorporate the flux into the solder itself such that the two step (flux then solder) process is reduced to a one step soldering process. U.S. Pat. No. 4,645,545 teaches the use of homogenously mixed solder and flux powders for the construction of solder preforms. However this approach has inherent limitations. First, the flux effectiveness is limited because a significant percentage of the flux may be trapped within the preform and is thus not available to adequately prepare the solderable surfaces until after the surrounding solder matrix has melted; at which time, the solder joint has already been made. In addition, the homogeneous distribution of flux and solder particles throughout the perform results in lower preform strength due to the non-load-bearing nature of the composite. For preforms with at least one small dimension, such as a thin ring for pipe applications, this limitation can render this material too weak to be practical.